1. Field of the Invention
The present invention relates to communication signal mixing and filtering systems and methods utilizing an encapsulated micro electro-mechanical system (MEMS) device. Furthermore, the invention is also directed to a method of fabricating a simple, unitarily constructed micro electromechanical system (MEMS) device which combines the steps of signal mixing and filtering, and which is smaller, less expensive and more reliable in construction and operation than existing devices currently employed in the technology.
Micro electromechanical system (MEMS) technology has been proposed for the fabrication of narrow band-pass filters (high-Q filters) for various communication circuits at frequencies below 200 MHz. Ordinarily, these filters employ the natural vibrational frequency of micro-resonators in order to be able to transmit signals at very precise frequencies, while concurrently attenuating signals and noise encountered at other frequencies.
In essence, communication carrier signals at radio frequencies (RF) are normally converted to intermediate frequencies (IF) for processing such as channel selection, signal isolation and the like. This particular conversion is generally implemented by mixing a carrier signal with the sinusoidal output of an oscillator in a non-linear device so that an output signal is generated which is either the sum of or the difference between the two input signals. A band-pass filter is then employed in order to select the desired converted intermediate frequency (IF) carrier signal for processing. Thereafter, a second conversion may be implemented in order to remove the intermediate frequency carrier and extract the final communication information; for instance, such as an audio message. The same two conversion steps may also be implemented in transmission in a reverse order; in effect, proceeding from an audio signal to the intermediate frequency (IF) carrier and then to the final communication radio frequency (RF) carrier frequency.
Basically, super-heterodyne communication transceivers depend upon precision electrical filtering and carrier signal mixing for signal processing at convenient intermediate frequencies. Generally these circuits possess three stages of operation. In a first stage, the radio frequency (RF) input signals are isolated utilizing a band-pass filter and then amplified. In a second stage, this signal is then beat against an intermediate frequency oscillator signal in order to reduce (or alternatively increase) its frequency for signal processing. In a third stage, after processing, the signal may then be further modulated with another oscillator signal in order to obtain audible frequencies for communication. These same stages may also occur in a reverse sequence in translating from audio frequencies to (RF) carrier transmission.
The present invention uniquely utilizes the above-mentioned first and second stages of operation, in which the carrier frequency is changed in order to perform various functions on the signal.
The conversion and filtering steps referred to hereinabove, can be implemented through the intermediary of pure electronic circuits; however, the resultant intermediate carrier is normally considered too broad in its frequency range for precision processing. Currently, the radio frequency (RF) filter is made with the excitation of an external crystal, commonly in a transmission mode. The intermediate frequency (IF) is filtration commonly attained with the use df external surface acoustic-wave (SAW) filters. The use of these two components which are ordinarily provided externally of the integrated circuit which is employed for signal amplification and processing, increases system complexity and adds to fabrication costs.
2. Discussion of the Prior Art
Typically, MEMS resonator filter devices are fabricated through the intermediary of standard integrated circuit masking/depositions/etching processes. For instance, specific details regarding the manufacture and structure of MEMS band-pass filters are readily disclosed in the following publications:
1) xe2x80x9cMicromachined Devices for Wireless Communicationsxe2x80x9d, C. T.-C. Nguyen, L. P. B. Katechi and G. M. Rebeiz, Proc. IEEE, 86, 1756-1768.
2) xe2x80x9cSurface Micromachining for Microelectromechanical Systemsxe2x80x9d, J. M. Bustillo, R. T. Howe and R. S. Muller, Proc. IEEE, 86, 1552-1574 (1998).
3) xe2x80x9cHigh-Q Micromechanical Oscillators and Filters for Communications xe2x80x9d. C. T.-C. Nguyen, IEEE Intl. Symp. Circ. Sys., 2825-2828 (1997).
4) A.-C. Wong, H. Ding, C. T.-C Nguyen, xe2x80x9cMicromechanical Mixer+Filterxe2x80x9d, Tech. Dig. of I.E.E.E./I.E.D.M., San Francisco, Calif., Dec. 198, pp 471-474.
Reverting to the foregoing publications, references (1 through 3) are primarily directed to the general field of utilizing various MEMS devices which are adapted to replacing communication elements. These publications are directed to the description of various conductors, filters and the like which have been constructed using micro-lithography and integrated circuit processing, and essentially are only of limited significance as representing technological background material with respect to the inventive concept.
A solution which is directed to solving the problem of carrier signal mixing and filtering employing micro electro-mechanical system (MEMS) devices concerning the aspects carrier signal mixing and filtering is disclosed in reference 4). This device consisted of two parallel clamped-clamped beam resonators (cantilevers), which have been coupled together with an insulating mechanical bridge. Both resonants are fabricated to possess a natural frequency IF. The input signal (RF) is capacitively coupled to one resonator, which in turn is electrically connected to a sinusoidal local oscillator (LO). The natural vibrational frequency of that resonator is RF-LO. Because of non-linear aspects of the resonator, the resonator mixes the incoming RF signal with the LO signal, and converts it into mechanical motion. This motion is mechanically coupled using the bridge, to the second resonator, which in turn is electrically connected to a DC bias. The mechanical motion induced in this resonator is then capacitively detected as the output signal. The fabricated device operates at a resonant frequency of 27 MHz. Because of the need for an insulating coupling beam between the two conducting resonators, the device is constituted of polysilicon, and then the beams doped using ion implantation. This increased the resistance of the beams considerably over that of metallic components, and consequently the increased insertion losses of the component when used in a circuit. Further, the device is about 20 xcexcmxc3x9720 xcexcm in size, rendering it difficult to encapsulate for protection against further IC processing.
In addition to the foregoing publications, prior patented devices which relate to electronic mixer-filters are set forth hereinbelow but which fail to provide a system and method utilizing the inventive MEMS device for communication signal mixing and filtering, in a manner analogous to that contemplated by the present invention.
For instance, Fraise U.S. Pat. No. 4,516,271, xe2x80x9cMicrowave Mixer with Recovery of the Sum Frequencyxe2x80x9d, concerns the use of a wave-guide cavity to mix and filter RF signals. The function of this device is similar to that of the present concept; however, it uses reflection of electro-magnetic waves to process the signal in contrast to the mechanical resonator used here.
Sakamoto U.S. Pat. No. 5,083,139, xe2x80x9cPulse Radar and Components Thereforxe2x80x9d, also mixes and filter RF signals using the interference of electromagnetic waves.
Scheinberg U.S. Pat. No. 5,563,545, xe2x80x9cLow Cost Monolithic GaAs Upconverter Chipxe2x80x9d, uses a standard xe2x80x9ctank-circuitxe2x80x9d consisting of inductors, capacitors and a variable resistor to achieve mixing and filtering of an RF signal.
Kennan U.S. Pat. No. 5,649,312 xe2x80x9cMMIC Downconverter for a Direct Broadcast Satellite Low Noise Block Downconverterxe2x80x9d, also uses standard electronic circuit components for mixing and filtering.
Abe. et al., U.S. Pat. No. 5,918,168, xe2x80x9cDouble Super Tunerxe2x80x9d, uses a dielectric layer for filteration, and references nine other U.S. patents which use similar techniques.
Finally, Berenz et al., U.S. Pat. No. 5,528,769, xe2x80x9cHigh Electron Mobility Transistor Monolithic Integrated Circuit Receiverxe2x80x9d, uses a xe2x80x9crat-ringxe2x80x9d circuit to generate the mixing of the input RF signal to its local oscillator, This technique is also a method using standard circuit components to accomplish the RF mixer-filter step.
None of the foregoing patents suggest using mechanical vibration for this process, and are therefore not applicable to the inventive concept.
Accordingly, in order to obviate the limitations and drawbacks encountered in the constructions and fabrications of various prior art MEMS system devices, the present invention combines the step of communication carrier signal mixing and filtering into a single and simply fabricated micro electromechanical system (MEMS) device. The MEMS system device enables both up-frequencies and down-frequencies conversions, and through the intermediary of a single unit, compatible with the incorporation thereof into an integrated circuit chip, encompasses many electronic components which are currently separately employed to perform the same functions, while concurrently being able to be signigicantly reduced in size, complexity and being extremely simple to manufacture, so as to render the MEMS device pursuant to the invention economical in nature, while highly reliable in the functioning thereof.
Accordingly, it is an object of the present invention to provide a novel micro electromechanical system (MEMS) device which combines the steps of communication carrier signal mixing and filtering.
Another object of the present invention resides in the provision of a single, unitarily constructed micro electromechanical system (MEMS) device combining the steps of communication carrier signal mixing and filtering which enables both up-frequency and down-frequency conversion.
A further object of the present invention is to provide a single micro electromechanical system (MEMS) device which is adapted to be incorporated into an integrated circuit chip, and which encompasses a multiplicity of electronic components within the device heretofore being separately provided.
Yet another, object of the present invention resides in the provision of a method for fabrication of the herein-described micro electro-mechanical system (MEMS) device adapted for the combining of communication carrier signal mixing and filtering.